Components of Net Periodic Pension and Postretirement Benefit Costs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension benefits
Components of net periodic cost
Service cost	$ 40	$ 38
Interest cost	66	74
Expected return on plan assets	(97)	(98)
Amortization of net actuarial loss (gain)	65	44
Settlement loss	10	10
Net periodic cost (credit)	84	68
Postretirement benefits
Components of net periodic cost
Service cost	4	3
Interest cost	8	9
Amortization of prior service credit	(6)	(6)
Amortization of net actuarial loss (gain)	(3)	(5)
Net periodic cost (credit)	$ 3	$ 1